More than 1,000 people have pleaded guilty or have been convicted on federal charges of defrauding the myriad COVID-19 relief programs that Congress established in the early days of the pandemic. And more than 600 other people and entities face federal fraud charges.

But that’s just the start, according to investigators scheduled to testify Wednesday to a congressional committee as House Republicans mark the beginning of what they promise will be aggressive oversight of President Joe Biden’s administration.

The House Committee on Oversight and Accountability is holding its first hearing in the new Congress on fraud and waste in federal pandemic spending. Congress approved about $4.6 trillion in spending from six coronavirus relief laws, beginning in March 2020 when Donald Trump was president.

“We owe it to the American people to get to the bottom of the greatest theft of American taxpayer dollars in history,” said Rep. James Comer, R-Ky., the committee’s chairman.

The Government Accountability Office is expected to tell the committee that the number of cases of suspected fraud is certain to grow in the coming months. For example, the inspector general for the Small Business Administration has more than 500 ongoing investigations involving loan programs designed to help businesses meet operating expenses during the pandemic. The inspector general for the Labor Department continues to open at least 100 unemployment insurance fraud investigations each week.

The GAO said the more than 1,000 convictions related to COVID-19 relief fraud are one measure of how extensive it was. How much money was lost to fraud? That’s unknown, the GAO said, but it reported in December that an extrapolation of Labor Department data would suggest more than $60 billion in fraudulent unemployment insurance payments during the pandemic. The GAO also warned that such an extrapolation has inherent limitations and should be interpreted with caution.

Still, lawmakers are anxious to discern how much theft has occurred and what can be done to stop it in future emergencies.

“We must identify where this money went, how much ended up in the hands of fraudsters or ineligible participants, and what should be done to ensure it never happens again,” Comer said.

Some 20 inspectors general work collaboratively to investigate pandemic relief spending. Michael Horowitz, who chairs a committee Congress created in March 2020 to lead oversight of COVID-19 spending, is also scheduled to testify.

In his prepared remarks, Horowitz said the committee issued a fraud alert this week regarding the use of more than 69,000 questionable Social Security numbers to obtain $5.4 billion in pandemic loans and grants.

Also testifying is David Smith, an assistant director of the Office of Investigations at the U.S. Secret Service, who predicts that efforts to recover stolen assets and hold criminals accountable for pandemic fraud will continue for years to come.